The present invention relates to a detecting device for detecting the presence of an item, such as sheets, envelopes, transported side by side.
There have been known image forming apparatuses such as copying machines and printers in which uppermost ones of two stacks of sheets contained side by side in a cassette are fed and transported simultaneously side by side to a photosensitive member and two toner images formed side by side on the surface of the photosensitive member are transferred to these two sheets.
In the above image forming apparatuses, when one of the sheet stacks in the cassette is used up or one of the sheets being transported side by side causes a jam along a transport path to the photosensitive member, the following problem occurs if the other sheet continues to be transported and the toner images are formed side by side. The toner forming the toner image corresponding to the side on which no sheet is being transported attaches to a transfer device and a transport path downstream from the transfer device, thereby staining the transport path and the transfer device and consequently a subsequently fed sheet. Further, when a sheet on either path causes a jam along a transport path extending from the photosensitive member to a discharge tray, it is necessary to detect the jam and remove the jammed sheet immediately.
Because of the above mentioned problems, it has been required to detect the absence of a sheet in one of the stacks in the cassette or an abnormal transport state of a sheet and to stop the feeding of the sheet and notify an operator of such an undesirable event.
In response to this requirement, sheet detectors have been provided individually in respective stacks of the cassette or along respective transport paths for the sheets. However, this arrangement necessitates as many sheet detectors as the transport paths, thereby complicating the construction of the image forming apparatus, and takes time and labor in wiring or the like.
Paying a special attention to that the feeding of sheets is stopped and the notification to the operator is made when the abnormality occurs along at least one of the transport paths along which sheets are transported, the following detecting device having a simplified construction is disclosed in Japanese Unexamined Utility Model Publication No. 2-86953. Specifically, this detecting device includes a plurality of contacting members, a plurality of pivotal members, and a photosensor. The contacting members are mounted on the same shaft and rotate according to the presence or absence of the sheet along the transport paths. The pivotal members as many as the contacting members are arranged side by side so as to move together with the corresponding contacting members. The photosensor includes a pair of a light emitter and a light detector which are arranged in such a manner that the pivotal members are held therebetween. With this detecting device, it can be detected using these contacting members, pivotal members, and photosensor whether the abnormality has occurred along at least one of the transport paths.
However, with the construction disclosed in the above reference, the number of pivotal members increases when the number of transport paths increases. Accordingly, it is necessary to space the light emitter and the light detector by a longer distance. This leads to the large size photosensor and therefore an increase in the production cost. Further, when the distance between the light emitter and the light detector becomes longer, these elements become more subject to the influence of the external light, thereby reducing the reliability of the photosensor.